application noteLise-Meitner-Straße 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax. +49 (0) 731 140 70200
www.witec.de, info@witec.deStress in Si
Combining Confocal Raman and Atomic Force Microscopy
Vickers Indent
The indentation hardness test as an elementof the hardness measurement makes it
possible to acquire a response curve for amaterial while measuring force andindentation depth. A special instrument, calleda nanoindentor was developed for this test.
With this instrument it is possible to produce
very small indentations with a depth of a few
microns and a force on the order of a few µN.
The advantage of this experiment is thelocalized deformation of the material thatenables the examination at small volumes.
For the indentation hardness test, a hard form

- a pyramid (Vickers indenter) in our
experiment - was pushed into the sample
material with a controlled force andpenetration.
In this study, the stress field around a
Vickers indent was analyzed using therelative shift of the 520/cm Si-Raman line.
The Vickers indent was performed with aforce of 50 mN, resulting in a pyramidal
hole of 210 nm depth and 2,75 µm diagonalsize, as shown in the AFM image in fig. 1(5 x 5 µm scale).
The images below show a comparision of
the topography (10 x 10 µm, 5 nm verticalscale, fig. 2) around the Vickers indent and astress image of the same area obtained in
Raman Imaging Mode ( 10 x 10 µm scale, fig.
3), with which a complete Raman spectrum
is acquired at every pixel, 100 x 100 pixels=10 000 spectra in this case. The Raman
image is provided by simply rotating theturret from the AFM objective to a standard
optical microscope objective (Nikon, 100 x
air, NA = 0.9 NCG) without moving thesample. As can be seen, the extent of thestress field can be detected more than 5 µm
from the center of the indent. The stressimage was obtained with 10 mW excitation
power (532 nm) and 70 ms integration time
per spectrum.

Fig. 1: AFM image, 5 x 5 µm scale.

+ 1.4/cm

0/cm- 1.7/cm

Fig. 2: AFM image: Topography around a Vickers indent,,
10 x 10 µm scaleFig. 3: Stress image of the same area as in fig. 2, obtained in theRaman Imaging Mode, 10 x 10 µm scale

Scale bar

application noteLise-Meitner-Straße 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700 Fax. +49 (0) 731 140 70200
www.witec.de, info@witec.de

The positions of the three cross sections(labelled 1-3) are marked in the stress image
in fig. 4 and displayed in fig. 5 to fig. 7.
As expected, the stress field has the symmetryof the Vickers pyramid. The tensile strain (bluein the stress image, fig.4) appears at thecorners of the pyramid, while the compressivestrain (yellow in the stress image, fig. 4)
appears along the flat sides.
The cross sections show the extreme precisionof the stress measurement with a standard
deviation of only 0,021/cm. This accuracy wasobtained by determining the peak positionof the parabolic fit of the Si-Raman line ineach measured spectrum. This degree ofprecision was achieved with an
integration time of 70 ms per spectrum.
As the relation between peak shift and stressfor silicon is 1 1/cm (Dw) = 435 MPa (s), an
accuracy of 0,02/cm corresponds to a stresssensitivity of only 9 MPa.

Fig. 5: Cross section, marked position 1 in fig. 4.

Fig. 6: Cross section, marked position 2 in fig. 4.

12Fig. 4: Stress image, obtainedin Raman Spectral Imaging
Mode, 10 x 10 µm scale.

3Fig. 7: Cross section, marked position 3 in fig. 4.
Samples courtesy of Helmut Fischer GmbH,
Sindelfingen, Germany

Si/SiGe strained epitaxial film on silicon
The second sample is a 50nm SiGe layer
on a silicon substrate with a 10nm cap
layer of epitaxial silicon. The lateralhomogeneity of strain in strained Si/SiGematerials is very important for the productionof high mobility IC that use these materials.
The SiGe layer has a certain lattice mismatch
to pure Si which depends on the Geconcentration. This strain relaxes partially
if the layer thickness exceeds the criticallayer
thickness. This critical thickness is
2 µm
not only a function of the Ge concentration,

but also a function of process temperatureand time. The lattice mismatch results inthe formation of misfit and associatedthreading dislocations which lead to thecharacteristic crosshatch pattern typicallyseen on the strained Si epilayer surface.
An example of this crosshatch pattern isshown in the AFM image of Figure 8a. Scan
range is 20 µm x 20 µm and the topography
scale is 30nm. The scan is rotated by 45
degrees with respect to the <110> directionsof the pattern. When performing a
Raman analysis of this structure, the2 µm

penetration depth of light into the structure
has to be taken into account. For thisexcitation wavelength (532nm), a penetration
depth of 760nm for pure silicon, and 540nm
for Si0.7Ge0.3 can be calculated.
Therefore, the main contribution comes fromthe Si substrate. This is clearly visible in Figure
8c, which shows a single Raman spectrum
obtained at this structure. The three peaks
corresponding to the SiGe layer: the Ge-Ge(200 1/cm to 300 1/cm), Si-Ge (380 1/cm to450 1/cm) and Si-Si (502 1/cm) stretchingapplication noteLise-Meitner-Straße 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700 Fax. +49 (0) 731 140 70200
www.witec.de, info@witec.demodes, are indicated, along with the Si-Si(520 1/cm) peak of the substrate. If the topsilicon layer is to be analyzed, a shorter
excitation wavelength with shallower
penetration depth can be used.
The stress in the partially relaxed SiGe
layer can be measured by analyzing thevariation of the peak position of the Si-Si

peak at 502 1/cm, which is shown in Figure
8b. Scan range is 20 µm x 20 µm with 100x 100 = 10,000 spectra. The measurement
conditions are identical to the example
above. The full range of the peak shift ofthe silicon Raman line is ± 1.5 1/cm. Fromthese values, a residual stress of up to 0.6GPa in the SiGe layer can be calculated.

Samples courtesy of Helmut Fischer GmbH,Sindelfingen, Germany
Fig. 8: (a) AFM image of the cross hatch pattern of a Si/SiGestrained epitaxial film on silicon.
Scan range is 20 µm x 20 µm and the topography scale is 30nm.
(b) Raman shift of the Si-Si stretching mode of the SiGe layer at502 1/cm. Scan range is 20 µm x 20 µm; all other measurement
parameters are as in Figure 3. Image scale is from 500.5 1/cm to503.5 1/cm.
(c) Single Raman spectrum. The three peaks corresponding tothe SiGe layer: the Ge-Ge (200 1/cm to 300 1/cm), Si-Ge (3801/cm to 450 1/cm) and Si-Si (502 1/cm) stretching modes areindicated, along with the Si-Si (520 1/cm) peak of the substrate.

2 µm

2 µm

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